The Stirling engine derives energy from a continuous external combustion process. All of the heat supplied from the combustion process has to be transferred through metal walls (heater tubes) to a pressurized hydrogen working fluid. The pressure-volume (P-V) and temperature-entropy (T-S) diagrams of the ideal Stirling cycle help in understanding the derivation of power for the engine. These diagrams (see FIGS. 6 and 7) illustrate that heat is transferred to the working fluid during the constant-volume phase 2-3 and during the isothermal expansion phase 3-4. Heat is rejected during the constant-volume phase 4-1 and during the isothermal compression phase 1-2. During the isothermal expansion of phase 3-4, heat addition occurs at the same rate at which work is produced by the fluid expansion. Therefore, to maintain maximum possible power out of the engine, the temperature of the working fluid must be maintained at a constant level and as high as possible, taking into consideration the metallurgical heat limit of the materials. Typically, a Stirling engine designed for automotive use is optimized for a hydrogen temperature of 710.degree. C. or higher.
An air/fuel control system is required to maintain such a constant hydrogen temperature. Such control system should also be capable of varying the ratio between air and fuel in response to a change in engine load, and also to provide a change in the air/fuel ratio as a function of fuel flow which may be varied as a result of exhaust gas recirculation. Air flow itself is a variable commodity since it is generated by a blower which is engine driven after the engine has been started. The air/fuel control system thus must respond to at least three superimposed parameters.
Varying the air/fuel ratio is necessary, apart from the desire to seek a constant hydrogen temperature, to control exhaust emissions and to improve engine efficiency. Unburned hydrocarbons in the exhaust, due to a rich fuel mixture, represent an energy loss; however, an air rich mixture results in less efficient heat transfer, and, therefore, a less efficient heating system. Varying amounts of exhaust gas recirculation (EGR) is required for dilution and to reduce the generation of nitrogen oxide emissions.
The prior art has attempted to provide an air/fuel control system for an automotive Stirling engine principally according to two concepts: (a) a closed loop system wherein the sensed hydrogen temperature was used to directly control a fuel metering device; or (b) an open loop system wherein a sensed change in the hydrogen temperature was utilized to control an air flow throttle valve which would modulate air flow, and then a fuel metering system was operated in response to a change in the air flow. The closed loop control system has proven deficient in spite of the fact that the fuel metering device employed dual pumps for improving the range of air/fuel ratios that could be administered. This resulted principally from low flow stability in the fuel injection rate range of 0.4-0.9 grams per second. Such system also required a motor which would operate the fuel injection device while operating at a constant low rpm; this was difficult to devise.
Open loop control systems have experienced comparable problems. One system employs a hydro-pneumatic fuel metering device responsive to an air flow measuring device consisting of a spring loaded flapper and a specially designed orifice. The flapper valve is located in the air inlet system between the air cleaner and the air throttle valve. The air flow signal is transmitted to a signal amplifier and it is designed so that the pressure drop in the device is proportional to the two-thirds power of air flow. This fuel metering assist is deficient because it is unable to compensate for the hysteresis of the open loop metering, and is not able to operate over a broad enough air/fuel range required of the engine. Another metering device typically used with the open loop system is a spool valve which in certain positions can bypass fuel. This latter device is not able to operate with a broad enough air/fuel ratio range.